Thank you very much for so useful information. Armed with your points, I have to get back to the shotgun again to look for other feathers that I have overlooked.
For this shotgun I can see vaguely a third fastner. It does not look like a Greene's crossbolt. I wonder if it is a Doll's head or Scott's crossbolt?
Both drilling and Holland Holland shotgus are exhibited at the Beijing Police Museum. There are other shotguns displayed there, foe example, a semiautomatic shotgun with Belgian proof marks on it. The former owners are all government and military high officials. I wonder if you would be so kind as to look at them?
With best wishes,
Haibin
